Generally, a spark plug used for ignition in an internal combustion engine (e.g., an automobile engine) includes a tubular metallic shell; a tubular insulator provided in an inner hole of the metallic shell; a center electrode provided in an inner hole of the insulator on the front end side thereof; and a ground electrode, one end of which is bonded to the front end of the metallic shell and the other end of which forms a spark discharge gap with the center electrode. In the case where such a spark plug is provided in a combustion chamber of an internal combustion engine, when spark discharge occurs at the spark discharge gap formed between the front end portion of the center electrode and the distal end portion of the ground electrode, a fuel charged in the combustion chamber is combusted.
Generally, an Ni alloy or the like is employed as a material for forming a ground electrode or a center electrode. Although an Ni alloy exhibits slightly poor oxidation resistance and erosion resistance, as compared with a noble metal alloy containing a noble metal (e.g., Pt or Ir) as a main component, an Ni alloy is suitable for use as a material for forming a ground electrode or a center electrode, since it is less expensive than a noble metal. However, in association with a recent tendency that higher temperature is required in a combustion chamber, when spark discharge occurs between a distal end portion of a ground electrode and a front end portion of a center electrode, each of the electrodes being formed of an Ni alloy or the like, spark erosion is likely to occur at the distal end portion of the ground electrode or the front end portion of the center electrode, the end portions facing each other. Thus, there has been proposed a method in which tips are provided on a distal end portion of a ground electrode and on a front end portion of a center electrode, the end portions facing each other, and spark discharge is caused to occur between the tips, to thereby improve the erosion resistances of the ground electrode and the center electrode. In many cases, these tips are formed of a material containing, as a main component, a noble metal exhibiting excellent oxidation resistance and erosion resistance.
For example, Japanese Patent Application Laid-Open (kokai) No. S59-160988 describes a spark plug “including tips formed of a Pt—Ni alloy material containing Ni (5 to 23 wt. %) and Pt (balance)” (see claim 2 of Japanese Patent Application Laid-Open (kokai) No. S59-160988), which spark plug is provided for achieving the following object; i.e., “to further improve the high temperature resistance of a spark plug including noble metal tips provided on spark discharge surfaces of a center electrode and an outer electrode” (see Japanese Patent Application Laid-Open (kokai) No. S59-160988, page 1, right column, lines 13 to 16).
Japanese Patent Application Laid-Open (kokai) No. S61-135083 describes “a spark plug including a bonded spark discharge portion formed of a Pt—Ni alloy, or a spark plug including a spark discharge portion formed of a Pt alloy exhibiting excellent erosion resistance, and an intermediate layer formed of a Pt—Ni alloy and provided between the Pt alloy and a matrix metal, characterized in that the Pt—Ni alloy is an alloy material containing Ni (5 to 40 wt. %), Pt (95 to 60 wt. %), and one or two metal oxides of Re, Y, Zr, Hf, Al, Ti, and La (0.02 to 1 part by weight), the metal oxide(s) being dispersed in the alloy material” (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. S61-135083), which spark plug is provided for achieving the following object; i.e., “to provide a spark plug exhibiting excellent durability and prolonged service life” (see Japanese Patent Application Laid-Open (kokai) No. 561-135083, page 2, upper left column, lines 11 to 13).
In recent years, an increasing demand has arisen for resource saving and cost reduction of spark plugs. Conceivable means for meeting such a demand is to reduce the amount of a noble metal (i.e., a rare and expensive resource) contained in a tip formed of the aforementioned noble metal alloy containing the noble metal as a main component. However, when the noble metal content of the tip is reduced, as described hereinbelow, a protrusion is formed on the tip due to various causes, and the protrusion may impede maintenance of erosion resistance and a spark discharge gap.
In view of the foregoing, an advantage of the present invention is to provide a spark plug including a tip provided on at least one of a center electrode and a ground electrode, which spark plug, even when the noble metal content of the tip is reduced as compared with conventional cases, exhibits erosion resistance and durability, since a spark discharge gap is maintained between the center electrode and the ground electrode.